Foamable styrenic polymer gel having a carbon dioxide blowing agent and a process for making a foam structure therefrom

The invention relates to a foamable styrenic polymer gel expandable to form a closed-cell foam structure with carbon dioxide blowing agent and a process for making the structure.

Due to present environmental concerns over the use of potentially ozone-depleting or flammable blowing agents, it is desirable to make styrenic polymer foam structures with a blowing agent containing carbon dioxide. Such foam structures blown with carbon dioxide can be seen in U.S. Pat. Nos. 4,455,272, 4,559,367, and European Patent Application 89114160.8.

A problem with using carbon dioxide as a blowing agent for styrenic polymer foam structures is its relatively low solubility in melts of the styrenic polymer. The low solubility results in high system pressure, which manifests itself in high pressure drop and excessive die pressure. Lowering pressure drop and die pressure would save energy costs, improve skin quality, and reduce the pressure duty requirements for process equipment. Low carbon dioxide solubility also results in small cell size and high density. Increasing carbon dioxide solubility would increase cell size and lower density, which in turn permits larger potential foam cross-section.

According to the present invention, there is a foamable styrenic polymer gel capable of forming a closed-cell foam structure, comprising: a) a flowable melt of a styrenic polymer material comprising greater than 50 percent by weight styrenic monomeric units and about 0.1 to about 40 percent by weight of oxygen-containing monomeric units based upon the total weight of the styrenic polymer material; and b) a blowing agent comprising about 5 weight percent or more carbon dioxide based upon the total weight of the blowing agent. The presence of the oxygen-containing comonomer in the melt of the styrenic polymer material enables process operating pressures to be significantly reduced when using carbon dioxide as a blowing agent compared to a corresponding melt of the styrenic polymer material without the oxygen-containing comonomer.

Further, according to the present invention, there is a process for making a styrenic polymer foam structure, comprising: a) heating a styrenic polymer material; b) incorporating into the melt polymer material at an elevated pressure a blowing agent to form a foamable gel; and c) expanding the foamable gel at a reduced pressure to form a foam structure. Preferably, the foamable gel is cooled to an optimum foaming temperature, and then expanded by extruding it through the die to form the foam structure. The blowing agent comprises about 5 weight percent or more of carbon dioxide based upon the total weight of the blowing agent. The styrenic polymer material comprises greater than 50 percent by weight styrenic monomeric units and about 0.1 to about 40 percent by weight of oxygen-containing monomeric units based upon the total weight of the styrenic polymer material.

The present foam structure comprises a styrenic polymer material. Suitable styrenic polymer materials include styrenic homopolymers and copolymers of styrenic compounds and copolymerizable ethylenically unsaturated comonomers. The styrenic polymer material may further include minor proportions of non-styrenic polymers. The styrenic polymer material may be comprised solely of one or more styrenic copolymers, a blend of one or more of each of styrenic homopolymers (such as polystyrene) and copolymers, or blends of any of the foregoing with a non-styrenic polymer. Regardless of composition, the styrenic polymer material comprises greater than 50 and preferably greater than 70 weight percent of styrenic monomeric units. Most preferably, the styrenic polymer material comprises a blend of a styrenic copolymer and a styrenic homopolymer. The styrenic polymer material preferably has a weight average molecular weight of 100,000-350,000 according to size exclusion chromatography.

Suitable styrenic monomers polymerizable to form styrenic polymers useful in the styrenic polymer material include those derived from alkenyl aromatic compounds such as styrene, alphamethylstyrene, alphaethylstyrene, ethylstyrene, vinyl benzene, vinyl toluene, chlorostyrene, and bromostyrene.

In the present invention, it is desirable to select a styrenic polymer material having in melt form a relatively high carbon dioxide solubility. Increasing carbon dioxide solubility allows process pressures employed in making a foam structure to be reduced. Extruder pressure, mixer pressure, die pressure and pressure drop between an extruder or mixer and the die could be reduced.

The styrenic polymer material contains about 0.1 to about 40 weight percent, preferably about 0.3 to about 30 weight percent, and more preferably about 2 to about 20 weight percent of oxygen-containing monomer (OCM) monomeric units based upon the total weight of the styrenic polymer material. The OCM will be a compound other than the styrenic comonomer or compound, and will be copolymerized therewith. The OCM monomeric unit content of the styrenic polymer material increases the solubility of carbon dioxide in the styrenic polymer material relative to a corresponding styrenic polymer material without the OCM monomeric unit content.

Suitable OCM include compounds of the formula: ##STR1## where R 1 and R 2 are H (hydrogen) or C 1-8 alkyl groups. Suitable OCM include C 1-8 alkyl acids, esters, and anhydrides, such as acrylic acid, methacrylic acid, ethacrylic acid, methyl methacrylate, ethyl methacrylate, butyl methacrylate, maleic acid, itaconic acid, maleic anhydride, methyl acrylate, ethyl acrylate, butyl acrylate, propyl acrylate, methyl methacrylate, and vinyl acetate. Other suitable OCM include C 1-8 aliphatic alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol, neopentanol, n-hexanol, h-heptanol, and n-octanol. Preferred styrenic polymer materials are those having OCM content such as those containing styrene/acrylic acid copolymer, styrene/butyl acrylate copolymer, styrene/butyl methacrylate, and mixtures of polystyrene with any or all of the foregoing.

Minor amounts of other copolymerizable monoethylenically unsaturated compounds other than OCM such as ionomeric derivatives, C 2-6 dienes, and C 2-8 olefins may be employed. Examples of suitable copolymerizable compounds include acrylonitrile, vinyl alcohol, amides, butadienes, maleimides, vinylsiloxanes, pentadiene, hexadiene, ethylene, propylene, hexene, and octene.

The foam structure is generally formed by melting and mixing the styrenic polymer material to form a polymer material melt, incorporating a blowing agent into the polymer material melt to form a foamable gel, and extruding the foamable gel through a die to form the foamed structure. During melting and mixing, the polymers are heated to a temperature at or above the glass transition temperature of the polymer material. Melting and mixing of polymer material and any additives is accomplished by any means known in the art such as with an extruder, mixer, or blender. Likewise, the blowing agent is incorporated or blended into the polymer material melt by any of the same above-described means. The blowing agent is blended with the polymer material melt at an elevated pressure sufficient to prevent substantial expansion of the resulting plastic gel or loss of generally homogeneous dispersion of the blowing agent within the foamable gel. The blowing agent is incorporated into the melt in a weight proportion of between about 1 to about 30 parts and preferably from about 2 to about 18 parts per hundred parts of the polymer to be expanded. The blowing agent is preferably incorporated into the melt in a liquid, gas, or vapor form. The foam gel is preferably passed through a cooler or cooling zone to lower the gel temperature to an optimum foaming temperature. For styrenic polymers, typical optimum foaming temperatures range from 110° C. to 135° C. The cooled gel is then passed through the die into a zone of lower or reduced pressure to form the foam structure. The zone of lower pressure is at a pressure lower than that in which the foamable gel is maintained prior to extrusion through the die. The lower pressure may be superatmospheric or subatmospheric (vacuum), but is preferably at an atmospheric level.

The blowing agent employed in the present invention preferably comprises 5 weight percent or more, preferably 10 weight percent or more, and more preferably 20 weight percent or more carbon dioxide based upon the total weight of the blowing agent. The blowing agent may be composed entirely of carbon dioxide as well. It is advantageous to employ as much carbon dioxide as possible in the blowing agent while still making a desirable foam structure because carbon dioxide is economical and does not harm the environment.

Secondary blowing agents which may be utilized in combination with carbon dioxide include inorganic agents, organic blowing agents and chemical blowing agents. Suitable inorganic blowing agents include nitrogen, argon, water, air, and helium. Organic blowing agents include aliphatic hydrocarbons having 1-9 carbon atoms, fully and partially halogenated aliphatic hydrocarbons having 1-4 carbon atoms, and C 1-6 aliphatic alcohols. Aliphatic hydrocarbons include methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, and the like. Fully and partially halogenated aliphatic hydrocarbons include fluorocarbons, chlorocarbons, and chlorofluorocarbons. Examples of fluorocarbons include methyl fluoride, perfluoromethane, ethyl fluoride, 1,1-difluoroethane, 1,1,1-trifluoroethane (HFC-143a), 1,1,1,2-tetrafluoroethane (HFC-134a), pentafluoroethane, difluoromethane, perfluoroethane, 2,2-difluoropropane, 1,1,1-trifluoropropane, perfluoropropane, dichloropropane, difluoropropane, perfluorobutane, perfluorocyclobutane. Partially halogenated chlorocarbons and chlorofluorocarbons for use in this invention include methyl chloride, methylene chloride, ethyl chloride, 1,1,1-trichloroethane, 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124). Fully halogenated chlorofluorocarbons include trichloromonofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), trichlorotrifluoroethane (CFC-113), 1,1,1-trifluoroethane, pentafluoroethane, dichlorotetrafluoroethane (CFC-114), chloroheptafluoropropane, and dichlorohexafluoropropane. C 1-6 aliphatic alcohols include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentane, isopentane, neopentane, n-hexanol, and isohexanol. Chemical blowing agents include azodicarbonamide, azodiisobutyro-nitrile, benzenesulfonhydrazide, 4,4-oxybenzene sulfonyl-semicarbazide, p-toluene sulfonyl semi-carbazide, barium azodicarboxylate, N,N'-dimethyl-N,N'-dinitrosoterephthalamide, and trihydrazino triazine.

Preferred secondary blowing agents are isopentane, ethanol, and mixtures of the same.

Though the preferred process for making the present structure is an extrusion process, it is understood that the above structure may be formed by expansion of beads, which may be molded at the time of expansion to form structures of various shapes. Insulating panels formed from molded, expandable beads are commonly referred to as bead board.

The present foam structure may be unimodal or bimodal in cell size distribution. A unimodal cell size distribution is one in which the cells are of a generally uniform size throughout the foam structures, except for the skin regions in the case of extruded foam. A bimodal cell size distribution is one in which there is one group of relatively larger primary foam cells of generally uniform size and another group of relatively smaller secondary foam cells of generally uniform size ranging in average cell size from about 5 percent to about 50 percent of the average cell size of the primary cells. The secondary cells may be situated within the cell walls or struts of the primary cells, or may be situated outside of or adjacent to the primary cells individually or in groups of two or more. A strut is a juncture of three or more cell walls. The primary cells may be generally dispersed throughout the secondary cells such that the foam structure has a generally heterogeneous dispersion of the two cell types throughout. Bimodal structures may be formed by employing a blowing agent having about 1 percent or more water based upon the total weight of the blowing agent. Unimodal structures may be formed when employing a blowing agent having water provided the water content in the blowing agent is sufficiently small. The blowing agent employed in making the present foam structure may contain water or may be free of water. Additional teachings directed to foam structures with bimodal cell distributions and processes for making are seen in U.S. Pat. Nos. 4,455,272 and 4,559,367, U.S. Ser. Nos. 07/895,970 filed Jun. 9, 1992 and 07/896,025 filed Jun. 9, 1992, and European Patent Application No. 89114160.8, which are incorporated herein by reference.

The foam structure may contain additional additives such as pigments, fillers, antioxidants, nucleating agents, extrusion aids, nucleating agents, stabilizing agents, antistatic agents, fire retardants, acid scavengers, or the like.

The foam structure is closed-cell, and has a closed-cell content of at least 90 percent according to ASTM D-2856-87.

The present foam structure is substantially non-crosslinked or substantially free of crosslinking. It is permissible however, for the present structure to have a slight degree of crosslinking which may occur naturally without the use of crosslinking agents or radiation.

The foam structure preferably has density of about 16 to about 80 kilograms per cubic meter according to ASTM D-1622. The foam component further preferably has an average cell size of about 0.05 to about 2.4 millimeters according to ASTM D3576-77.

The foam structure may be used to insulate various surfaces by applying to the surface an insulating panel fashioned from the present structure. Such panels may be used in any conventional insulating application such as roofing, buildings, refrigerators, etc.

The foam structure may also be formed into a plurality of discrete foamed particles for conventional loose-fill cushioning and packaging applications.

The following are examples of the present inventions and are not to be construed as limiting. Unless otherwise indicated, all percentages, parts, or proportions are by weight. Unless otherwise indicated, the term "pph" means parts per hundred by weight based upon the weight of a polymer or mixture of polymers.

Closed-cell styrenic polymer foam structures were made according to the process of the present invention in the following examples.

Certain physical properties were measured as follows: density according to ASTM D-1622; cell size according to ASTM D3576-77; open cell content according to ASTM D2856-87; environmental dimensional change (E.D.C.) according to ASTM C-578-83; and water vapor permeability (WVP) according to BS-4370 (Method 8).